(a) Field of the Invention
The present invention relates to an eight-speed automatic transmission. More particularly, the present invention relates to an eight-speed automatic transmission that includes a center supporter that may smoothly supply hydraulic pressure to clutches, and an input shaft that has a small diameter.
(b) Description of the Related Art
A typical shift mechanism of an automatic transmission utilizes a combination of a plurality of planetary gear sets. A powertrain of such an automatic transmission changes rotating speed and torque received from a torque converter of the automatic transmission and transmits the changed torque to an output shaft.
It is well known that when a transmission realizes a greater number of shift speeds, speed ratios of the transmission can be more optimally designed and therefore a vehicle can have better fuel mileage and better performance. For that reason, an automatic transmission that enables more shift speeds is under constant investigation.
In addition, with the same number of speeds, features of a powertrain such as durability, efficiency in power transmission, and size depend a lot on the layout of combined planetary gear sets. Therefore, designs for a combining structure of a powertrain are also under constant investigation.
A manual transmission that has too many shift-speeds causes inconvenience of excessively frequent shifting operations to a driver. Therefore, the positive features of more shift-speeds are more important for automatic transmissions because an automatic transmission automatically controls shifting operations basically without needing manual operation.
In addition to various developments regarding four and five speed powertrains, an eight speed automatic transmission has recently been developed.
FIG. 1 and FIG. 2 is a schematic diagram of an eight-speed automatic transmission according to a prior art.
In the above scheme, hydraulic pressure is supplied to frictional elements via a hydraulic line formed in an input shaft 3.
An eight-speed automatic transmission, as shown in FIG. 1 and FIG. 2, is provided with three planetary gear sets, four clutches, two brakes, and a plurality of hydraulic lines. In order to supply hydraulic pressure through the input shaft to the frictional elements, the diameter of an input shaft has to be increased so that the size of other elements has to be increased.
The increase of the diameter of the input shaft and the size of the elements leads to increased size and weight of an automatic transmission.
In addition, the hydraulic pressure supplied through the input shaft may be reduced and delay of response can occur because of the hydraulic pressure passages through the rotating elements.
In a case of the conventional eight-speed automatic transmission, as shown in FIG. 1 and FIG. 2, a second clutch C2 and a fourth clutch C4 are disposed between a first planetary gear set PG1 and a second planetary gear set PG2, and the second planetary gear set PG2 and a third planetary gear set PG3 respectively, and the scheme of the automatic transmission is complex, so that it is difficult to form an alternate hydraulic line.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.